Antimicrobial susceptibility testing of Bacteroides species by disk diffusion: The NordicAST Bacteroides study

Objectives: - Antimicrobial susceptibility testing (AST) of anaerobic bacteria has until recently been done by MIC methods. We have carried out a multi-centre evaluation of the newly validated EUCAST disk diffusion method for AST of Bacteroides spp. Methods: - A panel of 30 Bacteroides strains was assembled based on reference agar dilution MICs, resistance gene detection and quantification of cfiA carbapenemase gene expression. Nordic clinical microbiology laboratories (n 1/4 45) performed disk diffusion on Fastidious Anaerobe Agar with 5% mechanically defibrinated horse blood (FAA-HB) for piperacillin-tazobactam, meropenem and metronidazole. Results: - A total of 43/45 (95.6%) laboratories carried out disk diffusion per protocol. Intraclass correlation coefficients were 0.87 (0.80-0.93) for piperacillin-tazobactam, 0.95 (0.91-0.97) for meropenem and 0.89 (0.83-0.94) for metronidazole. For metronidazole, one media lot yielded smaller zones and higher variability than another. Piperacillin-tazobactam and meropenem zone diameters correlated negatively with cfiA expression. A meropenem zone diameter of <28 mm in B. fragilis indicated presence of cfiA. Piperacillin-tazobactam had the most false susceptible results. Categorical errors for this antimicrobial were particularly prevalent in cfiA-positive strains, and piperacillin-tazobactam had the highest number of comments describing zone reading difficulties. Conclusions: - Inter-laboratory agreement by disk diffusion was good or very good. The main challenges were media-related variability for metronidazole and categorical disagreement with the reference method for piperacillin-tazobactam in some cfiA-positive strains. An area of technical uncertainty specific for such strains may be warranted. & COPY; 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Automated summary

This study evaluated the EUCAST disk diffusion method for antimicrobial susceptibility testing of Bacteroides spp. and found good agreement between laboratories. However, there were discrepancies in susceptibility results for piperacillin-tazobactam in some cfiA-positive strains compared to the reference method.